Semiconductor Fundamentals · Topic 3 of 13
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Energy bands
Electrons in a solid occupy allowed energy bands separated by forbidden gaps.
What you’ll learn
- What the valence and conduction bands are
- How bands form from atomic orbitals
- How band occupancy sets material behaviour
Why it matters
Band structure explains, in one picture, why materials are conductors, insulators, or semiconductors.
Explanation
When atoms bond into a solid, their discrete energy levels broaden into continuous bands. The highest band that is full of electrons is the valence band; the next band up, largely empty, is the conduction band. Electrons must reach the conduction band to move freely.
In metals these bands overlap, so electrons flow easily. In insulators and semiconductors a gap separates them; the gap is large for insulators and small enough for semiconductors that some electrons can cross it.
Visual explanation
Key terminology
- Valence band
- The highest energy band normally filled with electrons.
- Conduction band
- The band electrons must enter to conduct.
- Fermi level
- The energy level with 50% occupation probability.
Example
Common mistakes
Watch out for:
- Picturing bands as single lines — they are dense ranges of closely spaced levels.
- Forgetting that an electron leaving the valence band leaves behind a mobile 'hole'.
Real-world application
Band engineering (e.g. strained silicon, heterojunctions) is used to boost transistor performance.
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Energy bands
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Bandgap
Semiconductor Fundamentals
The energy gap between the valence and conduction bands — the number that defines a semiconductor.
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